How to decrease vertex and Face ?
I would say the best thing you could do would be to decrease the amount of subdivisions on faces which are less likely to be seen, So for instance the chain, yes they play a valuable role in telling the story of the tank but when a texture is added you don't need as many faces. 

So just think of spaces that people wont be able to see. Like the back part and the inside or even underneath it. But great bloody Model mate, I can see you like Battlefield 1.

Also for the pins/bolts/rivets along the tank, are they spheres? If so change them to Hemisphere's.